The present invention relates to an image forming apparatus in which a toner image formed on the surface of a rotating image forming body is transferred onto a transfer sheet sent by a register roller at a transfer position opposed to a transfer charger.
In the above-described image forming apparatus, transfer conditions such as discharging voltage of the transfer charger at which the maximum transfer efficiency can be obtained, etc., change when the resistance of the transfer sheet changes. Accordingly, when transfer conditions are constant, there is a problem in which the transfer efficiency is lowered due to the change of resistance of the transfer sheet. In order to solve the problem, the following image forming apparatus has been disclosed in Japanese Patent Publication Open to Public Inspection No. 34834/1979. A transfer sheet conveyance means, such as a register roller, by which the transfer sheet is sent to a transfer position, is used for an electrode from which current flows in the direction of the thickness of the transfer sheet or in the direction of conveyance of the transfer sheet so that the resistance of the transfer sheet can be measured. According to the measured information, charging conditions of the transfer charger or a separation charger are determined so that high transfer efficiency or reliability of sheet separation can be stably obtained, independent of change of moisture content. Further, the following image forming apparatus has been disclosed in Japanese Patent Publication Open to Public Inspection 14271/1981. A pick-up roller for sending the transfer sheet stacked in a sheet feed cassette is used for an electrode from which current flows in the direction of conveyance of the transfer sheet and the direction of the length, perpendicular to the direction of the conveyance of the transfer sheet so that the resistance of the transfer sheet can be measured. According to the measured information, charging conditions of the transfer charger or the separation charger are determined.
In the resistance measuring method disclosed in Japanese Patent Publication Open to Public Inspection No. 34834/1979, since the transfer sheet is not pinched when electrode rollers stop, the resistance of the transfer sheet is measured during the conveyance of the transfer sheet. Accordingly, a varied, unstable measured value is obtained as shown by amplifier outputs V2 and V4 in FIG. 5 (which will be described later). Further, in this method, when it is required to correct the measured value using a reference value which is obtained under the same conditions as the resistance measurement of the transfer sheet so that the influence due to deterioration with time of the electrode rollers or the amplifier, etc., is excluded, it can not be corrected because the reference value is obtained after the the measured value has been obtained. Accordingly, satisfactory measurement accuracy can not be obtained.
Further, in the resistance measuring method disclosed in Japanese Patent Publication Open to Public Inspection 14271/1981, considerably local resistance between two points on the surface of the stacked transfer sheet is measured. Accordingly, when the contact position of the electrode rollers with the transfer sheet changes due to the conveyance of the transfer sheet, etc., then, an easily variable, unstable measured value is obtained. Therefore, in this method, the measured value can not be corrected by the reference value, and adequate measuring accuracy can not be obtained. Further, in this method, when voltage of about 500V is not impressed upon the pick-up roller, the output of the measurement becomes too small, so that this method is dangerous. Further, in this method, there is a problem in which a measuring apparatus for resistance of the transfer sheet is necessary for each sheet feeding means.
That is, in the image forming apparatus disclosed in both patent publications, even when charging conditions of the transfer charger or the separation charger are determined according to the resistance of the transfer sheet, sufficient transfer efficiency or separation efficiency can not be obtained.